Most aircraft ground taxi by using thrust produced by their main engine(s). As a result of the ground taxi speeds being relatively low the engines must be run at low power. This means there is relatively high fuel consumption as a result of the poor propulsion efficiency at this low forward velocity. This leads to an increased level of both atmospheric and noise pollution locally around airports. Even when the engines are run at low power it is generally necessary to apply the wheel brakes to prevent excessive ground taxi speeds, which leads to a high degree of brake wear particularly as the brakes tend to be cold prior to take-off. It is also to be noted that reversing a civil aircraft by using its main engines is not permitted.
Several autonomous taxi systems have been proposed in recent years for driving the wheels whilst the aircraft is on the ground. Any benefits from an autonomous taxi system must consider the fuel cost to the operator of aircraft flying with the extra mass of the ground taxi system. This means that the only practical solution is one that absolutely minimises the installed system mass. Most recent interest in autonomous ground taxi systems has focused on installing drive motors on the landing gear wheel axle. These proposed systems have several potential drawbacks.
Since brakes are installed within the wheels of aircraft main landing gear, this region is very congested and so it is difficult to install drive motors on the wheel axle of main landing gear. Therefore, development of these proposed systems has generally been limited to the nose landing gear. Nose landing gear support low vertical load (approximately 5% of the aircraft weight) during ground taxi operations, which could lead to traction problems when the wheels are driven. This is particularly the case when the aircraft centre of gravity is towards its aft limit and when the ground surface is slippery, e.g. when wet or icy.
Most landing gear have a shock-absorbing main leg with a sprung part attached to the aircraft and an un-sprung part which carries the wheel(s). Any drive system which is mounted on the wheel axle (i.e. on the un-sprung part of the landing gear) will increase the un-sprung mass of the landing gear. An increase to the un-sprung mass of the landing gear is undesirable from a dynamic response perspective where this could result in high loads being induced into the landing gear and aircraft structure. Significant strengthening of the landing gear and/or aircraft structure may be required to support these load increases. Furthermore the un-sprung landing gear components experience significantly higher vibration and acceleration loads than the sprung parts. For example, the accelerations on the sprung parts may be around 3.5 g to 5 g, whereas the accelerations on the un-sprung parts may be around 50 g to 60 g. Therefore, any drive system which is fixedly mounted on the un-sprung parts will need to be very robust. This is unlikely to lead to a mass optimised solution.
Furthermore, any drive system which is installed coaxial with the wheel axle cannot easily be removed. Not only is this an important consideration for maintenance purposes but studies have indicated that the benefits of even lightweight autonomous taxi systems may only be observed for the shorter range operations where an aircraft spends a higher proportion of its time taxiing. It would therefore be desirable to be able to quickly install and remove the majority of the autonomous taxi system equipment from the aircraft so as to optimise the aircraft economics in the event of longer range operations. This is generally not possible with an integrated drive system installed coaxial with the wheel axle.
U.S. Pat. No. 3,762,670 describes a landing gear wheel drive system including a pair of drum members arranged to move into friction driving engagement with the periphery of the tyres of the landing gear wheels for driving the wheels whilst the aircraft is on the ground. The drum members are mounted on a rotating shaft and a motor is connected to the shaft via a gearing mechanism. Most of the weight of these parts is supported by the sprung part of the landing gear main leg. Whilst this document addresses some of the short-comings of the prior art axle mounted wheel drive systems, the roller pressures on the tyre necessary to move the aircraft are considered to be so high as to lead to unacceptable damage to the tyre surface. In addition, the shaft carrying the drum members is highly loaded in bending and therefore will need to be massive or it will suffer fatigue. Therefore, this design is also unlikely to lead to a mass optimised solution, necessary for practical application.